This invention relates to on-axis spatial light modulators (SLMs) and optical processing, and combinations of SLMs to form all-optical correlators.
Optical correlators are dedicated optical computers capable of determining the degree of similarity between two or more images. Optical correlators can be used in, for example, fingerprint identification, machine vision, image processing, and on-line product evaluation in a manufacturing process. Optical correlators are attractive because of their ability to parallel process information at the speed of light.
Practical optical correlators require the capability for both dynamic input and dynamic filtering. To perform these tasks, spatial light modulators (SLMs), which are programmable masks, are used to imprint two-dimensional information (i.e., an image) onto a coherent optical wavefront. SLMs are typically electronically addressable devices, e.g., liquid crystal displays, which pose a variety of input/output bottlenecks, performance limitations, and packaging problems.
Optically addressable SLMs (OASLMs) typically offer higher resolution and can handle higher information transfer rates by taking advantage of the inherent parallelism found with optics. OASLMs have been limited by the available materials for their manufacture.
One class of materials for use in OASLMs is photochromic materials. These materials are capable of changing color upon exposure to radiant energy such as light. One such material is rhodopsin, or "visual purple," a photosensitive, red protein pigment in the retinal rods of marine fishes and most higher vertebrates, e.g., octopus, mollusks, and man. A protein related both in structure and function to rhodopsin is the halobacterial retinal-containing protein, bacteriorhodopsin (bR), which is a light-absorbing protein synthesized by the bacteria Halobacterium halobium. The chromophore is a retinal moiety linked via a protonated Schiff base near the middle of helix G to lysine-216.
In its natural state the bR molecules perform the biological function in the halobacterial cell of converting light into an electrochemical ion gradient across the membrane, i.e., it pumps cytoplasmic protons across the membrane to the outside of the cell, to synthesize ATP from inorganic phosphate and ADP. When irradiated, individual bR molecules undergo light-induced structural changes that result in large changes in optical properties as the molecule passes through different states in a so-called "photocycle."
In the initial B state of bR, also called the "light adapted" state, the retinal chromophore is in an all-trans molecular configuration. The B state has an absorption maximum at 570 nm, with a broad absorption band of +/-100 nm, that allows bR to be excited by means of light in the red, yellow, or green portions of the optical spectrum. Once a photon is absorbed, the retinal chromophore undergoes configurational and conformational changes, the first of which is in a sub-picosecond time frame. The chromophore subsequently goes through a series of short-lived intermediates to the so-called M state, which has an absorption maximum at 410 nm wavelength.
The M state can revert to the initial B state via thermal relaxation processes or by photochemical processes upon excitation with blue light (410+/-50 nm). The thermal relaxation of the chromophore from the M state is initiated by the reprotonation of the aspartic acid in position 96 (Asp-96 residue). The retinal molecule is then able to isomerize once again and relax to the all-trans B state. The lifetime of the M state depends on the kinetics of the reprotonation process, and can be altered by different means such as controlling the extent of drying, controlling pH, changing the temperature, and by modifying the molecular structure by genetic mutation. For example, Chen et al., Appl. Opt., 30:5188 (1991) describes high pH bR films in which the M state lifetime is increased from milliseconds to tens of seconds. The time required to switch between the M and B states via a photochemical process is much faster than the states' lifetime, and is typically on the order of microseconds. Because of the short lifetimes of the other intermediate states, the bR photocycle can be approximated by a two-state model that includes only the B and M states.
Of the rhodopsins, bR is the most chemically and environmentally robust (it can be stored for years without degrading) and, unlike many biological materials, bR is not adversely affected by environmental perturbations such as heat, light, and humidity. In fact, bR is stable at temperatures of up to about 140.degree. C., is stable with respect to photodegradation, and can be exposed to light for long periods of time without sacrificing optical performance. For example, no noticeable change is observed after a bR film is switched between the B and M states more than a million times with a quartz lamp with appropriate color filters.
In addition to its stability, bR has many desirable optical properties. Bacteriorhodopsin has a high optical absorption cross-section for both B and M states and attains optical saturation at moderate intensities (e.g., 100 mW/cm.sub.2). Bacteriorhodopsin additionally has a fast switching time, and can switch between M and B states in a matter of microseconds. Due to its optical properties, bR has been proposed as a material with applications in photonics technology, e.g., in information processing (Chen et al., Applied Optics, 30:5188, 1991; Korchemskaya et al., Sov. Journal of Quantum Electronics, 17:450, 1987), and in computer memories (Birge et al., Scientific American, p. 90, March, 1995).
The use of bR in optical image processing is based on the fact that bR's absorption of light triggers a photochemical cycle in the bR molecule. In its initial state, the bR film is isotropic with a random but rigid orientation of bR molecules. Illumination of a bR film at a wavelength of about 570 nm (B to M state transition) reduces the absorption coefficient of the film at that wavelength, and bleaches these molecules. Upon illumination of the film with a linearly polarized light (a so-called "actinic beam"), the film shows anisotropic properties of photoinduced dichroism and photoinduced birefringence (Burykin et al., Opt. Commun., 54:68, 1985). The magnitude and sign of the induced anisotropy are dependent on the actinic light intensity and wavelength. Since the actinic beam is linearly polarized and the bR molecules in the film are randomly oriented but rigidly maintained, only those bR molecules that have their transition dipole moments oriented for absorption in or near the electric field direction of the light are bleached. Turning off the actinic beam returns the film to its initial isotropic state with a relaxation time equal to the lifetime of the intermediate M state.
One practical concern when using OASLMs is the subsequent separation of the addressing beam or "write beam" from the coherent optical wavefront or "read beam," on which the image information is imparted. To address this concern, in many cases, only one of the read and write beams are oriented "on-axis," i.e., normal to the SLM, hence the read and write beams are spatially separated from one another subsequent to the SLM, even though they spatially overlap in the plane of the SLM. In other cases, where both of the read and write beams are oriented on-axis and are collinear with one another, a filter is used to block the wavelengths of the write beam and transmit the wavelengths of the read beam. For example, see, Imam et al., Opt. Lett., 20:225 (1995). In these cases, therefore, the wavelengths of the write beam must be different than the wavelengths of the read beam.